Localized pH Pulses in PBS Buffer Repeatedly Induced by Visible

Article Cite This: J. Phys. Chem. B XXXX, XXX, XXX−XXX

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Localized pH Pulses in PBS Buffer Repeatedly Induced by Visible Light Adnan Elgattar, Nawodi Abeyrathna, and Yi Liao*

J. Phys. Chem. B Downloaded from pubs.acs.org by KAROLINSKA INST on 01/12/19. For personal use only.

Florida Institute of Technology, Melbourne, Florida 32901, United States ABSTRACT: The pH of biological systems is important for the activity of enzymes, and abnormal cellular pH is related to many diseases. Spatial and temporal modulation of pH with light will be useful for studying the pH effects on enzymatic functions and disease mechanisms and may lead to new drug delivery and therapeutic methods. However, the pH of biological systems is maintained by pH buffers, which implies that only temporary pH change (pH pulse) can be induced in an open system. A key fundamental problem is whether a photoinduced pH pulse can be strong and long enough to generate a significant effect. In this work, a photoinduced pH pulse in a micrometer hydrophilic film in PBS buffer has been demonstrated. The thin film was made of an metastable-state photoacid (mPAH) polymer. It is an open system that allows exchange of protons. A quick release of the protons from the mPAHs and the proton exchange between the film and PBS resulted in a pH pulse generated by moderate visible-light irradiation. The magnitude of the pulse is 1.4−1.9 units with maximum pH change occurring after ∼18 s of the irradiation. Since the mPAH is a reversible photoacid, the pH pulse could be repeatedly generated after the photoacid recovered in the dark. This work shows that photochemical modulation of pH is possible even in buffered solutions.

1. INTRODUCTION A suitable pH of a biological system is critical for the activity of enzymes, and abnormal cellular pH is related to many diseases including cancer, cardiovascular diseases, Alzheimer’s disease, etc. A method that allows spatial and temporal modulation of pH with light will be useful for studying the pH effects on enzymatic functions and disease mechanisms and may lead to new drug delivery and therapeutic methods. It is worth mentioning that photoinduced proton transfer at the molecular level has been extensively studied using ultrafast spectroscopy.1,2 Our goal is to controllably produce a large proton concentration with measurable pH change and significant macroscopic effects. Metastable-state photoacids (mPAHs) can produce a large proton concentration with high efficiency and good reversibility under visible light.3 Reversible pH change over two units has been demonstrated previously.4 Over the past several years, mPAH has become a useful tool for controlling various proton transfer processes with light. Applications of mPAHs in energy conversion,5−7 sensor,8,9 polymerization,10 patterning,11−13 nanomaterials,9,14,15 molecular machines,16−18 photochromic materials,13,19−21 fragrance release,22 and biomedical23,24 areas have been reported by our group and other groups. For example, Gray, Dougherty, and co-workers showed that ion channels associated with vision and pain can be reversibly activated with light using an mPAH.23 The Li group demonstrated that a chloroplast entrapped with an mPAH produced 2.9 times more ATP than natural chloroplast.5 Chumbimuni-Torres’ group developed a calcium biosensor by incorporating mPAHs in polymer thin films.8 The Su group in © XXXX American Chemical Society

collaboration with our group showed that the photoacidity of an mPAH was enough to kill drug-resistant bacteria and assist the antibacterial activity of colistin.24 Although the potential of mPAHs is clearly demonstrated in these works, no photoinduced pH change in a pH buffer has been reported. Nonbuffered solutions of mPAHs were often used for modulating pH with light. The challenge is apparent for the buffer is against the pH change. However, since all biological systems are in pH buffers, it is expected that a localized pH change in a pH buffer is required for most biomedical applications. In an open system that allows proton exchange, the photoinduced pH drop will be neutralized by the buffer, which will result in a pH pulse. Since mPAH is reversible, another pulse can be generated after the photoacid recovers in the dark. The key is that the pH pulse must be large and long enough to generate a useful effect. In this work, we show that pH pulses of 1.4−1.9 units can be repeatedly generated in a micrometer film of a hydrophilic mPAH polymer in PBS.

2. RESULTS AND DISCUSSION PBS buffer (1×, pH = 7.4) was chosen since it is commonly used for biological study. To produce a localized pH change, a high local concentration of mPAH is required. Therefore, a copolymer of a indazole mPAH and hydroxyethyl methacrylate Received: December 4, 2018 Revised: January 4, 2019 Published: January 7, 2019 A

DOI: 10.1021/acs.jpcb.8b11677 J. Phys. Chem. B XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry B

Scheme 1. (a) Photoreaction of an Indazole mPAH, (b) Synthesis of the Photoacid Polymer P-mPAH, and (c) Synthesis of the Indicator Polymer P-MR

1 (5 wt %) and HEMA via an AIBN initiated radical polymerization. The stable amide linker between the polymer and the mPAH avoids hydrolysis in either basic or acidic conditions. The loading of the mPAH on the polymer was about 5 wt %. We found it was difficult to achieve a higher loading using this monomer. When 10 wt % of the monomer 1 was used in the polymerization, the resultant polymer contained less than 5 wt % of the photoacid. It is likely that the bulky photoacid sterically hinders the polymerization since it is close to the reactive acryloyl group. A thin film of P-mPAH was prepared by drop coating a solution of the polymer in methanol on a glass substrate. The polymer is insoluble in PBS buffer and, thus, does not require cross-linking. The film was dried on a heat plate (plate temperature 70 °C) for 15 min and then kept in a ventilation hood overnight. It was soaked in water for 15 min and then in PBS for 30 min. The sample was then put in a quartz cell filled with PBS and studied by UV−vis spectroscopy. The UV−vis spectrum of the film showed a strong absorption peak at 438 nm (Figure 1). Upon irradiation with a 470 nm LED from the

(HEMA) was synthesized as Scheme 1. Poly(hydroxyethyl methacrylate) is a well-known biocompatible hydrophilic polymer. It has been used as a host material for the preparation of mPAH thin films in previous work.25 Although phenolic mPAHs are most commonly used for different applications, they release a significant amount of protons without irradiation in PBS due to relatively high dark acidity.26 Our group reported previously that an mPAH with indazole and benzothiazolium moieties released its proton in PBS under visible light but not in the dark.26 (Scheme 1) Therefore, the indazole mPAH monomer 1 was synthesized from 6-amino-2methylbenzothiazole and then copolymerized with hydroxyethyl methacrylate to yield the mPAH polymer (P-mPAH). As shown in Scheme 1, 6-amino-2-methylbenzothiazole was reacted with acryloyl chloride to yield the acrylamide 2, which was then reacted with propane sultone in THF to yield compound 3. Compound 3 was coupled with indazole-7carboxylaldehyde via a Knoevenagel reaction using ammonium acetate as the catalyst, which gave the monomer 1 as an orange precipitate. P-mPAH was then synthesized by copolymerizing B


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Figure 1. UV−vis spectra of a P-mPAH film in PBS before and after irradiation (a) and UV−vis spectra of a P-MR film in water before after protonation with HCl (b).

top of the cell, the 438 nm peak was diminished and absorption near 320 nm substantially increased indicating the formation of the acidic product26 (Scheme 1). To demonstrate the photoinduced pH change, a copolymer of methyl red (MR 5 wt %) and HEMA was synthesized (Scheme 1). MR was chosen as the indicator because it has a pKa of 5.1 and is suitable for measuring pH between 6 and 4. As described below, the photoinduced pH in the photoacid film fell in this range. Another reason is that part of the absorption band of the protonated MR (MRH+) does not overlap with the absorption band of the indazole mPAH. As shown in Figure 1, a thin film of the MR polymer (P-MR) in water has a strong absorption band centered at 420 nm. After addition of HCl, the absorption peak of the protonated P-MR shifted to 501 nm. While P-mPAH has no significant absorption above 550 nm, the protonated P-MR substantially absorbs light between 550 and 600 nm, which allows us to observe the protonation of MR by the photoacid. It is worth mentioning that it is necessary to use the polymers instead of molecules of the photoacid and the indicator to avoid leakage during the tests. A thin film containing 80 wt % of P-mPAH and 20 wt % of P-MR was prepared by drop coating a methanol solution of the mixture on a glass slide and dried on a heat plate. The thickness of the film was measured to be 8 μm. After being soaked in water and then PBS, the sample was put in a quartz cell filled with PBS and studied by UV−vis spectroscopy. The setup is illustrated in Figure 2. This setup allows us to monitor the change of UV−vis absorption while the sample is under irradiation. The sample in PBS was irradiated by a 470 nm LED from the top of the cell. The photon flux at the center of the sample was ∼425 μmol s−1 m−2 (11 mW/cm−2) measured by a quantum meter. UV−vis spectra were recorded every 6 s. As shown in Figures 2 and 3, upon irradiation the absorption at 438 nm quickly decreased due to the photoreaction of the mPAH. In the meantime, the absorption between 500 and 600 nm increased indicating the formation of MRH+. The

Figure 2. UV−vis spectra of a film of P-mPAH and P-MR in PBS before irradiation and after 6, 18, and 72 s under irradiation. Experimental setup for monitoring the absorption change during the irradiation (inset).

absorption of MRH+ at 550 nm maximized at ∼18 s and then gradually decreased with time even though the sample was still under irradiation (Figure 3). The absorption at 438 nm kept decreasing after 18 s until the irradiation was stopped at 72 s. The results show that the proton concentration in the thin film increased upon irradiation and reached its maximum at ∼18 s, after which the proton concentration decreased although more acids were generated from mPAHs. Upon irradiation, the mPAHs were quickly converted to their acidic state and released protons. The released protons acidified the film and protonated MR. In the meantime, the H+/OH− exchange between the film and PBS basified the film. In the first 18 s, the acidification is faster than basification, which resulted in an increase of MRH+. After 18 s, the basification was faster than acidification even though more C


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Figure 3. Absorption of the P-mPAH/P-MR film at 550 nm (a) and 438 nm (b) during and after irradiation.

acids were generated from the mPAHs. This is likely due to two reasons. The rate of acid generation decreased with time due to a decreased amount of the mPAHs. In addition, when the protons were transferred from the photoacids to MR or PBS, the photoacids became anions and formed salts with MRH+ or the cations from PBS, which increased the hydrophilicity of the film and consequently accelerated the H+/OH− exchange. The two factors led to the decrease of MRH+ in the film after 18 s. On the basis of the absorbance change at 438 nm (Figure 3), we estimated that ∼3 × 10−2 M of mPAHs had changed to the acidic form and released their protons in the ∼8 μm film during the first 18 s. As described below, the pH in the film was ∼5.5 at 18 s. The proton concentration was ∼3 × 10−6 M, which was 4 orders of magnitude lower than the concentration of the acidic form of the mPAH. This indicates that most of the protons were quickly neutralized by the buffer soaked in the film before irradiation and diffused into the film during irradiation. Since the mPAH is reversible, the pH pulse can be reproduced after the mPAH recovers in the dark. To test this, the sample was kept in the dark for 8 h in PBS. The absorption at 438 nm grew back from 0.546 to 1.223 confirming the recovery of most of the photoacid (Figure 4). The sample was then irradiated for a minute. The maximum absorption at 550 nm (MRH+ absorption) during the irradiation was 0.112, which was lower than that of the first irradiation (0.143). The absorption at 438 nm decreased to 0.529 after irradiation. The sample was kept in the dark again for 8 h. The absorption at 438 nm went back to 1.152, and the absorption at 550 nm was 0.035, which was close to the value after the first irradiation (0.036). The sample was then irradiated for the third time. The maximum absorption observed at 550 nm was 0.108, and absorption at 438 nm went down to 0.535 after irradiation. To finish the third cycle, the sample was kept in the dark for 8 h. The absorption at 438 nm went back to 1.100, and the absorption at 550 nm was 0.035. The results of the second and the third irradiation were very close showing the reversibility of the system. It is also worth mentioning that the maximum absorption at 550 nm was observed at the third (18 s) or fourth scan (24 s) during

Figure 4. Changes of the absorption of the P-mPAH/P-MR film at 550 nm (up) and 438 nm (down) during the irradiation and recovery cycles.

the irradiation in all three tests. (As shown in Figure 3, the absorption values at the third and the fourth are very close.) To quantify the pH change induced by irradiation, the very same sample that was tested in the above experiments was immersed in a pH 6 buffer and kept in the dark for 15 min. The absorption at 550 nm increased from 0.035 to 0.104, which is close and slightly lower than the maximum absorption at 550 nm observed during the second and third irradiation and is significantly lower than that of the first irradiation. The pH 6 buffer was then replaced by a pH 5.5 buffer. The 550 nm absorption reached 0.143, which is about the same as the maximum absorption during the first irradiation. These results show that the first irradiation produced a pH pulse with a magnitude of ∼1.9 units and a minimum pH at 5.5, and the following irradiations produced smaller but consistent pH pulses of ∼1.4 unit with a minimum at ∼6.0. D


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3. CONCLUSION In this work, a photoinduced pH change in an mPAH polymer film in PBS has been demonstrated. The hydrophilic polymer film is an open system that allows exchange of protons. A quick release of the protons from the mPAH and proton exchange between the PBS and the film resulted in a pH pulse. The magnitude of the pulse is 1.4−1.9 units with maximum pH change occurring after 18 s of the irradiation. Since the mPAH is a reversible photoacid, the pH pulse can be repeatedly generated after the photoacid recovered in the dark. This work shows that photoinduced pH change based on mPAH could be applied to biological systems even though their pH is maintained by pH buffers.

10.75 (s, 1H), 8.99 (s, 1H), 8.55 (d, 1H, J = 15.6 Hz), 8.37 (d, 1H, J = 9.2 Hz), 8.30 (d, 1H, J = 7.2 Hz), 8.29 (s 1H), 8.25 (d, 1H, J = 16 Hz), 8.05 (d, 1H, J = 8 Hz), 7.86 (d, 1H, J = 9.2 Hz), 7.32 (t, 1H, J = 7.6 Hz), 6.50 (dd, 1H, J = 10 Hz), 6.36 (d, 1H, J = 15.2 Hz), 5.88 (d, 1H, J = 10 Hz), 5.13 (d, 2H, J = 7.6 Hz), 2.69 (d, 2H, J = 6 Hz), 2.25 (m, 2H, J = 6.4 Hz). Synthesis of Photoacid Polymer P-mPAH. Photoacid monomer 1 (20 mg), 2-hydroxyethyl methacrylate (380 mg), and AIBN (2 mg) were dissolved in 1.5 mL of DMSO. After the mixture was thoroughly purged with nitrogen, it was heated at 60 °C overnight. After it cooled to room temperature, the resultant viscous solution was added dropwise to diethyl ether to precipitate out the polymer product (0.35 g). The loading percentage of mPAH was measured by UV−vis spectroscopy. A methanol solution of the polymer with known weight/vol concentration was prepared, and its UV−vis spectrum was taken. The molar concentration of the mPAH on the polymer was calculated by dividing the absorbance at 438 nm by the extinction coefficient of the photoacid (2.9 × 10−4 L mol−1 cm−1). The weight of the mPAH unit was then calculated from the molar concentration, the solution volume, and the molecular weight of the mPAH monomer. The wt % was the weight of the mPAH unit divided by the weight of the polymer used for preparing the solution. 1H NMR (400 MHz, d6DMSO, δ ppm): δ = 7−9 (m, mPAH peaks, weak and broad, difficult to integrate), 4.79 (b, 1H), 3.86 (b, 2H), 3.54 (b, 2H), 1.75 (b, 2H), 0.90 and 0.73 (b, 3H). Synthesis of 4 (2-((4-((2-Hydroxyethyl)(methyl)amino)phenyl)diazenyl)benzoic acid). A mixture of anthranilic acid (0.5 g, 3.6 mmol), concentrated HCl (0.5 mL), and water (1 mL) was placed in an ice bath and then was diazotized by adding a cold solution of sodium nitrite (0.25 g, 3.6 mmol, 0.5 mL water) dropwise; then, a cold solution of 2-(Nmethylanilino)ethanol (0.82 g, 5.4 mmol, 1 mL water) was added to the cold diazonium salt. After that, the reaction mixture was stirred for 15 min in an ice bath; then, a solution of sodium acetate (0.52 g in 1 mL water) was added, and stirring was continued for 1 h. The mixture was kept in a refrigerator for 12 h, and then, it was left for 2 h at room temperature. A solution of NaOH (2 mL, 20%) was added into the reaction mixture. Consequently, it was left at room temperature for 1 h. The product was filtered and washed many times with water and acetic acid (3 mL, 10%) to remove unreacted 2-(N-methylanilino)ethanol. The dried product material was collected to give a dark-red powder in 65% yield. 1H NMR (400 MHz, d6-DMSO, δ ppm): δ = 13.17 (s, 1H), 7.75 (d, 2H, J = 8.8 Hz), 7.71 (d, 1H, J = 2.4 Hz), 7.61 (d, 1H, J = 2.4 Hz), (t, 1H, J = 2.4 Hz), 7.48 (m, 1H), 6.88 (d, 2H, J = 9.2 Hz), 4.80 (t, 1H, J = 5.2 Hz), 3.60 (q, 2H, J = 5.6 Hz), 3.56 (t, 2H, J = 5.2 Hz), 3.09 (s, 3H). Synthesis of 5 (2-((4-((2-(Acryloyloxy)ethyl)(methyl)amino)phenyl)diazenyl)benzoic acid). The mixture of 4 (0.45 g, 1.50 mmol) and triethylamine (0.3 g, 3.00 mmol) was dissolved in anhydrous THF (10 mL). A solution of acryloyl chloride (0.27 g, 3.00 mmol) in anhydrous THF (1.5 mL) was added dropwise to the above solution with temperature kept below 10 °C. The reaction was then stirred overnight at ambient temperature. The solvent was evaporated under reduced pressure, and the crude product was purified by column chromatography using dichloromethane and ethyl acetate (9:1) as the eluent to obtain a dark red product, yield 0.10 g (20%). 1H NMR (400 MHz, d6-DMSO, δ ppm): δ = 13.12 (s, 1H), 7.76 (d, 2H, J = 9.2 Hz), 7.74 (d, 1H, J = 2 Hz),

4. EXPERIMENTAL SECTION Unless otherwise noted, reagents and solvents were commercially available and used as received without any further purification. UV−vis spectra were obtained from a Varian Cary 60 Scan UV−vis spectrophotometer. NMR spectra were determined in deuterated solvents on a Bruker av400 NMR spectrometer. Chemical shifts were reported in delta (δ) units, parts per million (ppm) downfield from TMS. The light sources for irradiation were 470 nm LED arrays with 120 LEDs purchased from www.theledman.com. Photon flux was measured by an apogee quantum meter. 4.1. Synthesis of P-mPAH and P-MR (Scheme 1). Synthesis of 2 (N-(2-Methylbenzothiazol-6-yl)acrylamide). The starting material 6-methyl-2-aminobenzothiazole (1.50 g, 9.1 mmol) was dissolved in dry dichloromethane (18 mL). Then, triethylamine (1.00 g, 10.0 mmol) was added, and the mixture was stirred for 5 min. Next, the mixture was cooled in an ice bath, and acryloyl chloride (0.87g, 9.60 mmol) was added dropwise. The reaction mixture was stirred for 24 h at ambient temperature and then poured into water. The product was extracted with dichloromethane, and the organic layers were dried with MgSO4. After the solvent was evaporated under reduced pressure, the resultant crude product was purified by silica gel column chromatography using 5:3 hexane/ethyl acetate as the eluent to obtain a light-yellow solid (1.67 g, 84% yield). 1H NMR (400 MHz, DMSO, δ ppm): δ = 10.37 (s, 1H), 8.50 (s, 1H), 7.85 (d, 1H, J = 8.8 Hz), 7.56 (d, 1H, J = 8.8 Hz), 6.46 (dd, 1H, J = 10 Hz), 6.28 (d, 1H, J = 15.2 Hz), 5.78 (d, 1H, J = 10 Hz), 2.72 (s, 3H). Synthesis of 3 ((6-Acrylamido-2-methylbenzothiazol-3ium-3-yl)propane-1-sulfonate). A mixture of compound 2 (0.2 g, 1.00 mmol), 1,3-propane sultone (0.16 g, 1.40 mmol), and 1.00 mg of butylated hydroxytoluene (BHT) in ∼1 mL of THF was stirred and heated (plate temperature 90 °C) in a sealed vial for 4 days. The precipitate was washed with THF to yield the product (0.24 g, 78% yield). 1H NMR (400 MHz, d6DMSO, δ ppm): δ = 10.72 (s, 1H), 8.96 (s, 1H), 8.37 (d, 1H, J = 9.2 Hz), 7.86 (d, 1H, J = 9.2 Hz), 6.46 (dd, 1H, J = 10 Hz), 6.35 (d, 1H, J = 15.2 Hz), 5.86 (d, 1H, J = 10 Hz), 4.86 (t, 2H, J = 8.4 Hz), 3.15 (s, 3H), 2.63 (t, 2H, J = 6.4 Hz), 2.14 (m, 2H, J = 6.8 Hz). Synthesis of mPAH Monomer 1. Compound 2 (90 mg, 0.26 mmol), 1H-indazole-7-benzaldehyde (0.44 g, 3.0 mmol), a catalytic amount of ammonium acetate, and 1 mg of BHT were stirred in 1.5 mL of ethylene glycol and heated at 75 °C overnight. The orange precipitate was washed with THF and acetone to obtain a dark reddish orange solid (69 mg, 56%). 1 H NMR (400 MHz, d6-DMSO, δ ppm): δ = 13.95 (s, 1H), E



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7.61 (d, 1H, and t, 1H, J = 3.2 Hz), 7.48 (m, 1H, J = 3.2 Hz), 6.97 (d, 2H, J = 9.2 Hz), 6.28 (d, 1H, J = 16 Hz), 6.13 (dd, 1H, J = 10.4 Hz), 6.95 (d, 1H, J = 10.4 Hz), 4.33 (t, 2H, J = 5.2 Hz), 3.81 (t, 2H, J = 5.6 Hz), 3.08 (s, 3H). Synthesis of the Indicator Polymer P-MR. Monomer 5 (30 mg), 2-hydroxyethyl methacrylate (570 mg), and AIBN (3 mg) were dissolved in DMSO (1.5 mL). After the solution was degassed with nitrogen, it was heated at 60 °C overnight to obtain a viscous solution. The solution was added dropwise to diethyl ether to precipitate out the polymer product as a darkred solid polymer (530 mg). 1H NMR (400 MHz, d6-DMSO, δ ppm): δ 7.71, 7.58, 7.46, and 6.87 (b, MR peaks, 0.14 H), 4.78 (b, 1H), 3.86 (b, 2H), 3.53 (b, 2H), 1.75 (b, 2H), 0.90 and 0.73 (b, 3H). 4.2. Test of the Photoinduced pH Pulse in the PmPAH/P-MR Film. A mixture of 20 mg of P-mPAH and 5 mg of P-MR was dissolved in 0.5 mL of methanol assisted by ultrasonication. The solution was drop casted on glass slides and dried in a ventilation hood for 15 min at room temperature and then on a heat plate (plate temperature 70 °C) for 15 min. A reddish thin film was obtained and kept in the dark until it was tested. The thickness of the film was measured by a micrometer gauge with a resolution of 1 μm. Before the test, the thin film on a glass substrate was cut into small pieces that can fit into a regular quartz cell for a UV−vis spectrometer. The sample was soaked in water for 15 min and then in PBS for 30 min, after which the color of the film changed to orange. It was then put into a quartz cell filled with PBS with a tilt angle (Figure 2). The sample was irradiated from the top by a 470 nm LED array, and the UV−vis absorption change was monitored by a UV−vis spectrometer during and after irradiation. The photon flux of the irradiation (∼425 μmol s−1 m−2, 11 mW/cm−2) was measured by putting the detector of a hand-held quantum meter at the same distance to the LED as the center of the sample. It is worth mentioning that although PBS does not absorb 470 nm light, the photon flux on the sample in PBS could be a little lower than the measured value due to reflection and scattering. As described above, the sample was irradiated three times. Each irradiation lasted about 1 min, and the interval between the irradiation times was ∼8 h. To quantify the pH change, PBS was removed from the cell 8 h after the last irradiation. A pH 6 buffer (citric acid/Na2HPO4) was added slowly to the cell without moving the position of the sample. UV−vis spectrum was collected after the sample was kept in the new buffer for ∼15 min. The pH 6 buffer was then substituted by a pH 5.5 buffer, and the UV−vis spectrum was collected after the sample was kept in the new buffer for ∼15 min.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: yliao@fit.edu. ORCID

Yi Liao: 0000-0002-8217-8202 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Support from the National Science Foundation (1565613) is gratefully acknowledged. F


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Localized pH Pulses in PBS Buffer Repeatedly Induced by Visible

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